1. Field of the Invention
The present invention relates to a system and a method for using a surface acoustic wave (SAW) based radio frequency (RF) transmitter for amplitude modulation RF transmission in a tire pressure monitoring (TPM) system.
2. Background Art
It is known in the automotive industry to provide for wireless monitoring of vehicle tire parameters, particularly tire pressure. In some such tire pressure monitoring (TPM) systems, tire pressure sensors and radio frequency (RF) transmitters that can generate and transmit at least one amplitude modulated (AM) signal. In each tire, the tire pressure sensed by the tire pressure sensor is transmitted by the transmitter through an antenna to a receiver/controller located on the vehicle. The tire pressure information delivered to the receiver/controller by the RF AM signals from the transmitters is subsequently conveyed to a vehicle operator or occupant, typically using a display unit. In such a fashion, tire pressure monitoring systems can help to improve vehicle safety. Exemplary tire pressure monitoring systems are described and shown in U.S. Pat. Nos. 6,112,587 and 6,034,597.
Remote keyless entry (RKE) systems are also well known in the automotive industry. Some RKE systems can include a conventional RF AM transmitter used by the vehicle operator or occupant to transmit signals that control such functions as door, trunk, etc. locking/unlocking, turning on/off lights, sounding an alert, arming/disarming an anti-theft system, etc. and a receiver/controller in the vehicle that processes the transmitter control signals.
However, conventional AM transmitters that are not based on surface acoustic wave (SAW) technology (i.e., non-SAW AM transmitters) are limited to transmission of lower data rates than is desired for some tire pressure monitoring and RKE applications. Furthermore, some conventional SAW based AM transmitters have two or more transistors to generate the amplitude modulation. As a result, conventional SAW based AM transmitters are costly and can have significant circuit board size and weight. Yet further, many conventional tire pressure monitoring and RKE AM oscillator implementations are configured as on-off keyed (OOK) devices (i.e., devices only having two fixed percentages, usually 0% and 100%) AM capability.
Thus, there exists a need for a system and a method for a SAW based RF transmitter to provide AM signal modulation generation in a single transistor configuration. Such a system and method would generally provide fewer components and hence be less costly than conventional approaches. Such a system and method would generally provide the desired data rates for applications such as TPM and RKE applications. Such a system and method is generally adjustable for any desired value (or percentage) of AM capability as compared with many conventional AM oscillators that only implement OOK modulation.